time.h(0p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

time.h(0P)              POSIX Programmer's Manual             time.h(0P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       time.h — time types

SYNOPSIS         top

       #include <time.h>

DESCRIPTION         top

       Some of the functionality described on this reference page
       extends the ISO C standard. Applications shall define the
       appropriate feature test macro (see the System Interfaces volume
       of POSIX.1‐2017, Section 2.2, The Compilation Environment) to
       enable the visibility of these symbols in this header.

       The <time.h> header shall define the clock_t, size_t, time_t,
       types as described in <sys/types.h>.

       The <time.h> header shall define the clockid_t and timer_t types
       as described in <sys/types.h>.

       The <time.h> header shall define the locale_t type as described
       in <locale.h>.

       The <time.h> header shall define the pid_t type as described in
       <sys/types.h>.

       The tag sigevent shall be declared as naming an incomplete
       structure type, the contents of which are described in the
       <signal.h> header.

       The <time.h> header shall declare the tm structure, which shall
       include at least the following members:

           int    tm_sec   Seconds [0,60].
           int    tm_min   Minutes [0,59].
           int    tm_hour  Hour [0,23].
           int    tm_mday  Day of month [1,31].
           int    tm_mon   Month of year [0,11].
           int    tm_year  Years since 1900.
           int    tm_wday  Day of week [0,6] (Sunday =0).
           int    tm_yday  Day of year [0,365].
           int    tm_isdst Daylight Savings flag.

       The value of tm_isdst shall be positive if Daylight Savings Time
       is in effect, 0 if Daylight Savings Time is not in effect, and
       negative if the information is not available.

       The <time.h> header shall declare the timespec structure, which
       shall include at least the following members:

           time_t  tv_sec    Seconds.
           long    tv_nsec   Nanoseconds.

       The <time.h> header shall also declare the itimerspec structure,
       which shall include at least the following members:

           struct timespec  it_interval  Timer period.
           struct timespec  it_value     Timer expiration.

       The <time.h> header shall define the following macros:

       NULL          As described in <stddef.h>.

       CLOCKS_PER_SEC
                     A number used to convert the value returned by the
                     clock() function into seconds. The value shall be
                     an expression with type clock_t.  The value of
                     CLOCKS_PER_SEC shall be 1 million on XSI-conformant
                     systems. However, it may be variable on other
                     systems, and it should not be assumed that
                     CLOCKS_PER_SEC is a compile-time constant.

       The <time.h> header shall define the following symbolic
       constants. The values shall have a type that is assignment-
       compatible with clockid_t.

       CLOCK_MONOTONIC
                     The identifier for the system-wide monotonic clock,
                     which is defined as a clock measuring real time,
                     whose value cannot be set via clock_settime() and
                     which cannot have negative clock jumps. The maximum
                     possible clock jump shall be implementation-
                     defined.

       CLOCK_PROCESS_CPUTIME_ID
                     The identifier of the CPU-time clock associated
                     with the process making a clock() or timer*()
                     function call.

       CLOCK_REALTIME
                     The identifier of the system-wide clock measuring
                     real time.

       CLOCK_THREAD_CPUTIME_ID
                     The identifier of the CPU-time clock associated
                     with the thread making a clock() or timer*()
                     function call.

       The <time.h> header shall define the following symbolic constant:

       TIMER_ABSTIME Flag indicating time is absolute. For functions
                     taking timer objects, this refers to the clock
                     associated with the timer.

       The <time.h> header shall provide a declaration or definition for
       getdate_err.  The getdate_err symbol shall expand to an
       expression of type int.  It is unspecified whether getdate_err is
       a macro or an identifier declared with external linkage, and
       whether or not it is a modifiable lvalue. If a macro definition
       is suppressed in order to access an actual object, or a program
       defines an identifier with the name getdate_err, the behavior is
       undefined.

       The following shall be declared as functions and may also be
       defined as macros. Function prototypes shall be provided.

           char      *asctime(const struct tm *);
           char      *asctime_r(const struct tm *restrict, char *restrict);
           clock_t    clock(void);
           int        clock_getcpuclockid(pid_t, clockid_t *);
           int        clock_getres(clockid_t, struct timespec *);
           int        clock_gettime(clockid_t, struct timespec *);
           int        clock_nanosleep(clockid_t, int, const struct timespec *,
                          struct timespec *);
           int        clock_settime(clockid_t, const struct timespec *);
           char      *ctime(const time_t *);
           char      *ctime_r(const time_t *, char *);
           double     difftime(time_t, time_t);
           struct tm *getdate(const char *);
           struct tm *gmtime(const time_t *);
           struct tm *gmtime_r(const time_t *restrict, struct tm *restrict);
           struct tm *localtime(const time_t *);
           struct tm *localtime_r(const time_t *restrict, struct tm *restrict);
           time_t     mktime(struct tm *);
           int        nanosleep(const struct timespec *, struct timespec *);
           size_t     strftime(char *restrict, size_t, const char *restrict,
                      const struct tm *restrict);
           size_t     strftime_l(char *restrict, size_t, const char *restrict,
                          const struct tm *restrict, locale_t);
           char      *strptime(const char *restrict, const char *restrict,
                          struct tm *restrict);
           time_t     time(time_t *);
           int        timer_create(clockid_t, struct sigevent *restrict,
                          timer_t *restrict);
           int        timer_delete(timer_t);
           int        timer_getoverrun(timer_t);
           int        timer_gettime(timer_t, struct itimerspec *);
           int        timer_settime(timer_t, int, const struct itimerspec *restrict,
                          struct itimerspec *restrict);
           void       tzset(void);

       The <time.h> header shall declare the following as variables:

           extern int    daylight;
           extern long   timezone;
           extern char  *tzname[];

       Inclusion of the <time.h> header may make visible all symbols
       from the <signal.h> header.

       The following sections are informative.

APPLICATION USAGE         top

       The range [0,60] for tm_sec allows for the occasional leap
       second.

       tm_year is a signed value; therefore, years before 1900 may be
       represented.

       To obtain the number of clock ticks per second returned by the
       times() function, applications should call sysconf(_SC_CLK_TCK).

RATIONALE         top

       The range [0,60] seconds allows for positive or negative leap
       seconds.  The formal definition of UTC does not permit double
       leap seconds, so all mention of double leap seconds has been
       removed, and the range shortened from the former [0,61] seconds
       seen in earlier versions of this standard.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       locale.h(0p), signal.h(0p), stddef.h(0p), sys_types.h(0p)

       The System Interfaces volume of POSIX.1‐2017, Section 2.2, The
       Compilation Environment, asctime(3p), clock(3p),
       clock_getcpuclockid(3p), clock_getres(3p), clock_nanosleep(3p),
       ctime(3p), difftime(3p), getdate(3p), gmtime(3p), localtime(3p),
       mktime(3p), mq_receive(3p), mq_send(3p), nanosleep(3p),
       pthread_getcpuclockid(3p), pthread_mutex_timedlock(3p),
       pthread_rwlock_timedrdlock(3p), pthread_rwlock_timedwrlock(3p),
       sem_timedwait(3p), strftime(3p), strptime(3p), sysconf(3p),
       time(3p), timer_create(3p), timer_delete(3p),
       timer_getoverrun(3p), tzset(3p), utime(3p)

COPYRIGHT         top

       Portions  of this text are reprinted and reproduced in electronic
       form  from  IEEE  Std  1003.1-2017,  Standard   for   Information
       Technology  --  Portable  Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018  Edition,  Copyright
       (C)   2018   by  the  Institute  of  Electrical  and  Electronics
       Engineers,  Inc  and  The  Open  Group.   In  the  event  of  any
       discrepancy  between  this  version and the original IEEE and The
       Open Group  Standard,  the  original  IEEE  and  The  Open  Group
       Standard  is  the  referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in  this  page
       are  most likely to have been introduced during the conversion of
       the source files to man page format. To report such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                        time.h(0P)

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